I had a similar issue with my 2011 27" a while back...it turned out to be interference form a wifi hub nearby. Can you run the Apple Hardware Test from your original installation media? That may give a clue as to whether or not your GOU is about to fail:

I had a similar issue with my 2011 27" a while back...it turned out to be interference form a wifi hub nearby. Can you run the Apple Hardware Test from your original installation media? That may give a clue as to whether or not your GOU is about to fail:

Does it do it straight from startup or does it get progressively worse the longer it's been on?

If it gets worse as it stays on it's probably over heating. Clear out any crap in the fan and if that doesn't help it might help to remove the heat sink assembly and replace the thermal paste (or get a 3rd party after market fan replacement, if any are available for your card)

If it's doing it all the time it's worth trying a different cable or screen to rule out those components.

^^^Many people on here baked their 8800 (including me) and in most cases it's resurrecting these cards for some time.
But your case looks like failed memory on card. Baking won't help IMO, but if card is trash already, you can always try.

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